1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor which electrically detects a distortion of a magnetostrictive film provided on a surface of a steering shaft, thereby detecting steering torque applied to the steering shaft, and a production method and an evaluation method for the same.
2. Description of the Related Art
Magnetostrictive torque sensors are used in electric power steering systems for vehicles, and the electric power steering system is a support system which, when a driver turns the steering wheel while driving the vehicle, has a motor cooperate therewith so as to assist the steering effort. In the electric power steering system, the magnetostrictive torque sensor detects the steering torque caused in the steering shaft coupled to the steering wheel by the driver turning the steering wheel. The electric power steering system controls the auxiliary steering force that is the output from the motor based on the detected steering torque and a vehicle speed signal from a vehicle speed sensor, which detects the speed of the vehicle, thereby reducing the steering effort of the driver.
A magnetostrictive torque sensor has been proposed where two magnetostrictive films of Ni—Fe alloy are provided one above the other on the surface of the steering shaft so as to have opposite directions of magnetic anisotropy (refer to, e.g., Japanese Patent Application Laid-Open Publication No. 2004-333449). When the steering torque acts on the steering shaft, the steering shaft is twisted, thus distorting the magnetostrictive films, and the alternating-current resistance and the like of coils provided around the magnetostrictive films vary according to an inverse magnetostriction characteristic due to magnetic anisotropy. The magnetostrictive torque sensor detects this change, thereby detecting the steering torque acting on the steering shaft.
As to, e.g., vehicles, forklifts, and the like, ambient temperature of use may be high. Due to, for example, heat from the engine such as a combustion engine or a motor, heat from a factory, and further heat from direct sunlight, ambient temperature of use reaches 80 to 110° C. In, particularly, vehicles used at such use ambient temperature, the auxiliary steering force may differ in magnitude between when turning the steering wheel rightward and when turning it leftward. In particular, after a vehicle is left for a long time at use ambient temperature, when turning the steering wheel while being in a stopped state, i.e., when performing so-called stationary steering, the driver may realize that the auxiliary steering force differs in magnitude between the rightward turning and leftward turning of the steering wheel. The cause for this is thought to be that the magnitude of the steering torque signal detected by the magnetostrictive torque sensor shifts at use ambient temperature to be different between the rightward turning and leftward turning of the steering wheel.